We have recently identified abnormalities of T cell subclasses (helper and suppressor cells) in a pilot study done on patients with hemophilia. These abnormalities may be induced by factor VIII infusion products and are similar to the acquired immune deficiency syndeome (AIDS) identified recently in homosexual patients. The aims of this study are to determine the prevalence of immunologic abnormalities in a large group of hemophilic patients. We will compare the abnormalities identified in our patients with those from other countries to examine the possibility that this is just an endemic problem within the United States. We will study the natural history of these abnormalities and determine whether the abnormalities are reversible over time. In patients with abnormal studies, we will study the possibility that a change in transfusion product might cause the abnormal results to revert to normal. The immunologic studies will include a blood count, platelet count, total T and B cells, helper cells (T4), suppressor cells (T8), killer cells (OKM1), platelet-associated IgG, and immune complexes. We are concerned that the immunologic abnormalities, which are similar to AIDS, may be due to a transmissible agent present in commercial factor VIII concentrate. This study will help define the extent of this problem and possible ways of altering treatment so as to reverse the abnormalities or prevent their development.